The disclosed seal is used in a high performance gas turbine engine as a shaft seal for the purpose of preventing gases at high temperature and pressure from entering the engine oil system. In a particular embodiment the seal operated successfully at shaft speed of 465 feet per second, gas pressure of 45 psia and gas temperature of 1200.degree.F. Under these conditions prior art seals, capable of preventing oil or air leakage, had very short lives. The present invention is an improvement over the prior art in that it utilizes a carbon ring which under normal operating conditions is subject to essentially no wear. The carbon seal ring is fabricated so that its inner diameter is initially larger than the cool seal runner size, but slightly smaller than the hot runner size. Therefore, the carbon seal ring initially presents a gap through which a controlled air leakage occurs. However, when the shaft diameter grows due to temperature and centrifugal forces during initial operation the carbon seal ring is worn by runner friction until its final size is equal to the hot seal runner diameter. Thus, after initial operation the carbon seal ring is perfectly conformed to the seal runner and has essentially no gap, only an oil bearing film is provided between the carbon seal ring and the shaft.
In summary, the resulting aperture of the carbon seal ring is self-adjusting during initial engine operation as a result of seal runner thermal and centrifugal growth. During subsequent engine operation and after complete conforming of the carbon seal ring, both wear and heat generation at the carbon seal ring are negligible. The carbon ring is fractured at one location to permit ring expansion sufficient to accommodate initial thermal growth of the shaft. In addition, the carbon ring is pinned to the housing through a single radial slot for anti-rotation of the carbon ring and to accommodate shaft eccentricity or run out. The combination of the single anti-rotation pin, the single crack or fracture in the carbon ring, and the frictional torque results in a couple which unwinds the carbon ring; this minimizes wear.